linux-kernel - Re: Showing /sys/fs/cgroup/memory/memory.stat very slow on some machines

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Date:   Wed, 18 Jul 2018 19:58:18 +0200
From:   Bruce Merry <bmerry@....ac.za>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>
Subject: Re: Showing /sys/fs/cgroup/memory/memory.stat very slow on some machines

On 18 July 2018 at 19:48, Shakeel Butt <shakeelb@...gle.com> wrote:
> On Wed, Jul 18, 2018 at 10:40 AM Bruce Merry <bmerry@....ac.za> wrote:
>> > Yes, very easy to produce zombies, though I don't think kernel
>> > provides any way to tell how many zombies exist on the system.
>> >
>> > To create a zombie, first create a memcg node, enter that memcg,
>> > create a tmpfs file of few KiBs, exit the memcg and rmdir the memcg.
>> > That memcg will be a zombie until you delete that tmpfs file.
>>
>> Thanks, that makes sense. I'll see if I can reproduce the issue. Do
>> you expect the same thing to happen with normal (non-tmpfs) files that
>> are sitting in the page cache, and/or dentries?
>>
>
> Normal files and their dentries can get reclaimed while tmpfs will
> stick and even if the data of tmpfs goes to swap, the kmem related to
> tmpfs files will remain in memory.

Sure, page cache and dentries are reclaimable given memory pressure.
These machines all have more memory than they need though (64GB+) and
generally don't come under any memory pressure. I'm just wondering if
the behaviour we're seeing can be explained as a result of a lot of
dentries sticking around (because there is no memory pressure) and in
turn causing a lot of zombie cgroups to stay present until something
forces reclamation of dentries.

Cheers
Bruce
-- 
Bruce Merry
Senior Science Processing Developer
SKA South Africa